Lehigh University
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Researchers achieve greater, more reliable laser power

Researchers at Lehigh and the University of Wisconsin-Madison have devised a way to boost the peak power of a semiconductor laser without triggering an increase in optical power density that often damages a laser's output structure.

The invention by Nelson Tansu, Luke Mawst and colleagues in industry utilizes a "narrow lateral waveguide" to broaden the laser beam.

The wider beam lowers the peak optical power density at the output facet of the laser, which in turn allows an increase in the maximum optical power that can be achieved from the diode lasers.

Tansu says the invention could find applications in telecommunications and in medical and military technology, all of which make use of high-power semiconductor lasers.

Tansu, Mawst and five members of Alfalight Inc. of Madison, Wisconsin, have been awarded a U.S. patent for the invention, which is titled "Narrow Lateral Waveguide Semiconductor Laser."

Tansu is assistant professor of electrical and computer engineering at Lehigh and a member of the Center for Optical Technologies. Mawst is associate professor of electrical and computer engineering at the University of Wisconsin.

"This technology will allow one to increase the maximum power achievable of single-mode lasers by spreading the mode," says Tansu. "This approach also leads to lower peak power density at the facet, which in turn should improve the reliability of the lasers."

Semiconductor, gas and solid-state lasers are the types of lasers most used by engineers. Of the three, says Tansu, semiconductor lasers, which can include gallium-arsenide, indium-phosphide, gallium-aluminum-arsenide and other compound semiconductors, are the most efficient and the most suitable for compact applications.

The maximum optical lasing power that can be achieved with a semiconductor laser, says Tansu, is often limited by high optical power density at the output facet where the laser originates. This excess density can melt the facet and lead to catastrophic optical mirror damage to the diode lasers.

By utilizing a narrow lateral waveguide, says Tansu, it is possible to spread the optical lasing mode field over a much wider region, while maintaining single mode operation. This lowers the peak optical power density at the output facet, while enabling a higher maximum optical power to be achieved from the diode lasers without melting the facet.

Although the narrow lateral waveguide requires more incoming electrical power to achieve lasing condition, says Tansu, it offsets this added expense by achieving greater maximum lasing power without damaging the output facet.

In addition to lower peak optical power density, says Tansu, the new invention also achieves single mode operation. And the laser is easily grown and processed by standard semiconductor crystal growth techniques - Metalorganic Chemical Vapor Deposition (MOCVD) and Molecular Beam Epitaxy (MBE).

MOCVD instruments in Lehigh's Center for Optical Technologies enabled researchers to fabricate semiconductor nanostructures as laser gain media with a precision of up to one or two monolayers, says Tansu. A monolayer, or plane of atoms in a semiconductor crystal, measures 2.8 angstroms in thickness, with an angstrom being one ten-billionth of a meter.

Tansu and Mawst received another U.S. patent last fall for a new method of achieving 1550-nanometer lasers on gallium-arsenide. The technique utilizes Type-II gallium-arsenide antimony/indium gallium-arsenide nitride quantum well optoelectronics to achieve 1550-nm lasers on gallium-arsenide and has potential uses in optical telecommunications.

Typically, lasers emitting at the 1550-nm wavelength regime are based on indium-phosphide technology, which requires more complex methods to fabricate VCSELs (vertical cavity surface-emitting lasers) and also requires expensive methods to ensure thermal stability.

By using the new patented technology on type-II quantum wells, says Tansu, scientists can potentially realize low-cost and high-performance 1550-nm lasers on GaAs capable of operating up to elevated temperatures.

Posted on Tuesday, April 05, 2005

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